This invention relates to the field of fluid pumps and particularly to pumps providing precise metering of relatively small volumes.
The technology of fluid pumping has its roots in antiquity. The piston pump, in which a piston of limited depth closely fits within a cylinder, with or without adapting packing, is an extremely old structure. Linear extension of a pump shaft translates the piston to displace a volume of fluid equal to the cross-sectional area moved and the length of the stroke. Displacement pumps, with which the instant invention is concerned, are a variant of the piston pump in which a piston of extended depth, and which does not closely fit the bore of the cylinder, moves into the cylinder and displaces a volume equal to the volume of the piston moved into the cylinder. Displacement pumps have the inherent advantage of having the packing placed at the end of the cylinder where it can be more effective and adjustable. Metering precision in both of the types of pumps described depends on precision of the stroke length.
U.S. Pat. No. 4,941,808 describe a multi-mode, differential fluid displacement pump which can provide at least two different measured doses. The pump has a chamber in which are mounted first and second diameter pistons which define first and second volumes for reciprocation within the chamber. Reciprocation means are provided to deliver predetermined volumes by movement of either or both of the two pistons. By using two fluid displacement pistons, accurate metering of a small quantity of sample and a larger quantity of reagent can be done in one unit. The elongated pistons of such a pump are aligned axially and move separately or together.
A problem known as flow hysteresis is known to occur when the multimode pumps are used to meter quite small and precise volumes. Flow hysteresis is evidenced when the pump shaft is extended a limited amount and the responsive volume delivered is less than predicted by simple calculation. This anomaly is due to the introduction of compressibility into a fluid system assumed to be non-compressible by the presence of one or more bubbles of gas (usually air) trapped at a dead space in the flow system.
It is, therefore, an object of the present invention to provide an improved displacement pump which provides more precise and predictable metered volumes by eliminating hysteresis caused by trapped gas bubbles.